Annealing and dopant activation are two processes commonly performed on semiconductor substrates in fabricating logic devices, memory devices, photoemission devices, energy devices, and the like. A semiconductor substrate, such as silicon, germanium, a silicon germanium alloy, or a compound semiconductor such as any of the group III/V, group II/VI, or CIGS semiconductor compounds known to the art, is doped with a selected dopant, or multiple dopants. The dopants are implanted or deposited and diffused into the semiconductor matrix. Upon implantation and/or diffusion, the concentration of dopants as a function of depth within the substrate assumes a certain profile, and the implantation/diffusion process typically disrupts the crystal structure of the semiconductor matrix. The dopant concentration profile is usually not optimal for device performance, and the disrupted, or in some cases fully amorphized, crystal structure increases resistivity of the substrate.
To adjust the concentration profile and repair the crystal structure, the substrate is annealed, during which process the dopants are encouraged to diffuse into a desired concentration profile and attach to the crystal matrix. The annealing process also moves the semiconductor atoms back into a matrix position, repairing the crystal structure of the substrate. The dopants are activated by their inclusion in the crystal matrix, enhancing the electrical properties of the substrate. Reduction of defects in the crystal matrix improves conductivity of the material.
The diffusion process is difficult to control. While dopants are diffusing into a more desirable profile, some dopants are diffusing outside the target doping zone, leading to undesirable and/or unstable properties such as current leakage and voltage drift. As device geometries shrink according to Moore's Law, the size of target doping zones becomes accordingly smaller, and controlling the diffusion of dopants during an anneal process becomes more challenging. Methods of using visible and IR radiation for fast annealing are currently used to achieve very fast anneals, reducing the background thermal energy propagating through the material and driving unwanted diffusion, but such methods are expected to reach an effective limit at technology nodes below about 22 nm.
Use of microwave energy for annealing has been demonstrated (see Splinter, et al., U.S. Pat. No. 4,303,455), but microwave annealing has never achieved large-scale commercial acceptance. One challenging aspect of microwave annealing, and annealing in general, is achieving uniform results. Achieving a uniform laser energy field for laser annealing has been the topic of considerable scholarship for decades, but methods and apparatus for uniform annealing of semiconductor substrates using microwaves are still elusive. Microwave annealing offers the possibility of non-thermal, or low thermal budget, radiative processing of substrates—that is, processing substrates using electromagnetic radiation that minimizes thermal energy propagating through the substrate, thus minimizing unwanted diffusion. Methods and apparatus are still needed, however, for exposing substrates to highly uniform microwave energy fields.